Investigators are trying to learn what set off an Army veteran of three tours in Iraq who left his base in the middle of the night and methodically massacred 16 Afghan civilians -- most of them children and women.

The attack comes just as outrage over the burning of several Korans by members of the U.S. military seemed to be calming down, but now there are new fears of retaliation against U.S. troops.

Local villagers responded with outrage, saying American forces are supposed to be there to protect them, not enter their homes and slaughter their families in the middle of the night.

Nine of the victims were children, and three were women, all shot while they slept in their beds, according to villagers and the Afghan president's office.

One grieving mother, holding a dead baby in her arms, said, "They killed a child, was this child the Taliban? Believe me, I haven't seen a 2-year-old member of the Taliban yet."

Afghan President Hamid Karzai called it "an assassination -- one that can not be forgiven."

U.S. officials were quick to condemn the attack.

"I offer my profound regret and deepest condolences to the victims and their families," Gen. John Allen, head of NATO forces in Afghanistan, said in a statement.

"This deeply appalling incident in no way represents the values of ISAF [International Security Assistance Force] and coalition troops or the abiding respect we feel for the Afghan people."

After the alleged shooting spree, it's believed the soldier returned to the base on his own, and calmly turned himself in. He remains in NATO custody. It's unclear whether the soldier knew the victims or whether the alleged attack was spontaneous and unprovoked. It's also unknown whether he had any accomplices.

The soldier's name has not been released, but a U.S. official told ABC News he is a 38-year-old staff sergeant who is married with two children, and served three tours in Iraq. This was his first tour in Afghanistan, where he has been since early December, the official said.

The shooting took place at 3 a.m. in two villages in the Panjwai district of southern Kandahar province, a hotbed for the Taliban insurgency against U.S .forces. The two villages are a short walk away from the U.S. base where the soldier was stationed.

Photos from the scene show blood-splattered floors and walls inside a villagers home, one of three believed to have been attached, and blood-soaked bodies of victims, including the elderly and young children, wrapped in blankets and placed in the backseat of a van. Some of the bodies appear to have been burned.

NATO has launched its own investigation, and Karzai has sent his delegation to Kandahar for its own inquiry.

President Obama "was was informed this morning of the incident by his senior national security staff and received a briefing from them early this afternoon before calling President Karzai," deputy National Security Council spokeswoman Caitlin Hayden said of the Afghanistan shooting. "This afternoon's meeting included National Security Adviser Tom Donilon, Deputy National Security Adviser Denis McDonough, and Special Assistant to the President for Afghanistan and Pakistan Doug Lute."